1. Field of the Invention
The present invention relates to an ink set and an ink cartridge for ink-jet recording, an ink-jet recording method, and an ink-jet recording apparatus.
2. Discussion of the Related Art
A typical ink-jet printer contains an ink set including multiple inks, for example, cyan, magenta, yellow, and black inks. Each of the inks includes a different colorant in a different amount. Therefore, each of the inks has a different thixotropic property, and viscosity of each of the inks has a different dependency on shearing speed. In other words, viscosity variation relative to shearing speed variation is different among the inks. It may be difficult to make viscosity difference small among multiple inks over the whole range of shearing speed. Accordingly, viscosity difference among multiple inks is generally made small only at low shearing speeds or at high shearing speeds.
When viscosity difference among multiple inks is large at high shearing speeds, satellites or ink mists are disadvantageously generated when the ink is discharged. A typical ink-jet printer discharges liquid ink droplets from nozzles of a recording head toward a recording medium to form an image thereon. It may occur that main droplets have long trails and the long trails separate from the main droplets as small droplets. Such small droplets are called satellites. Ink mists are formed in the same way as satellites, but are much smaller in size than satellites. Satellites are likely to degrade the resultant image quality, and ink mists are likely to cause faulty operation of printers.
When viscosity difference among multiple inks is large at low shearing speeds, the resultant images may blur because the multiple inks each have different permeation speeds through paper.
Japanese Patent Application Publication No. (hereinafter “JP-A”) 2007-146135 discloses an ink set containing multiple inks. The viscosity difference among the multiple inks is set to between 0 and 0.8 mPa·s at low shearing speeds. The viscosity is measured using an R-type viscometer from Toki Sangyo Co., Ltd. The measuring range of shearing speed is 320 (1/s) or less. There is no mention of the viscosity difference at high shearing speeds, but that might be large so long as the viscosity difference at low shearing speeds is small, because it may be difficult to reduce viscosity difference among multiple inks either at low shearing speeds or at high shearing speeds.
JP-A 2003-41172 also discloses an ink set containing multiple inks. The viscosity difference among the multiple inks is set to 0.5 mPa·s or less at high shearing speeds. There is no mention of the viscosity difference at low shearing speeds, but that might be large so long as the viscosity difference at high shearing speeds is small, because it may be difficult to reduce viscosity difference among multiple inks either at low shearing speeds or at high shearing speeds. Additionally, referring to Example 4 of the publication, the pigment concentration difference among the multiple inks is 0.5% at least, which may create viscosity difference among the multiple inks regardless of shearing speed.
JP-A 2006-282986 discloses an ink which comprises a fluorine-based surfactant for preventing image blurring. However, there is no mention of the relation between viscosity and shearing speed.